1. Field of the Invention
The present invention relates to an ion implantation method employed in the course of manufacturing semiconductors and, more particularly, an ion implantation method of neutralizing electrostatic charge induced on matters such as semiconductor wafers to be processed.
2. Description of the Related Art
The ion implantation system has been widely used to dope ions particularly to semiconductor wafers because the amount and depth of ions implanted can be controlled with high accuracy.
Positive ions are struck against a semiconductor wafer at high speed in the doping process in which the ion implantation system is used. Electrons are thus driven out of the wafer and positive charge is stored at the insulating layer (or insulated devices) on the wafer. The surface of the semiconductor wafer is thus charged positive and the insulating layer thereof suffers breakdown by discharge (the so-called dielectric breakdown or electrostatic discharge damage). As a result, the product yield of semiconductor elements is reduced accordingly.
In order to prevent the semiconductor wafer from being charged positive, electrons shower over the positive charge build up on the surface of the semiconductor wafer and neutralize positive charge on the wafer. This is the so-called electron shower system.
As shown in FIG. 1, in the case of the conventional electron shower system, an electron beam flooding system section 34 is arranged between an ion supply source (not shown) an semiconductor wafers W to flood electron beams 22. When the positive charge is transported by an ion beam 10 to the semiconductor wafers W, a filament 18 is heated by Joule heating in the section 34 to generate thermoelectrons. A reflector 24 is located on the backside of the filament 18 because the thermoelectrons are emitted from the filament 18 in all directions. Reverse bias voltage is applied from a power supply 26 to the reflector 24 and almost all of the thermoelectrons thus generated are reflected in a desired direction by the reflector 24 to be used as beam of primary electrons 22.
Voltage of minus 200 to 300V is applied from another power supply 20 to between the filament 18 and the guide 12. The thermoelectrons are thus accelerated to high speed primary electrons 22 and when these primary electrons strike the inner wall of the beam guide 12, secondary electrons of low energy are ejected from the wall and shower over the semiconductor wafers W on a rotating disk 14. As a result, the positive charge accumulated on the wafers by the ion beam 10 is neutralized by the secondary electrons compensated.
In the case of the conventional electron shower system, however, the diameter of the ion beam 10 is much smaller than that of the wafer W and smaller than the secondary electron shower. The secondary electrons shower over a quite wide area. This causes the secondary electrons to shower over that part of the wafer to which the ion beam 10 is not transported and the area thus results in negative charging. This negative charging of parts of the wafer W results in destruction of the devices on those parts of the wafer W.
When the conventional neutralizing system described above is employed, therefore, both of positive and negative charging cannot be prevented to the whole of the surface of the wafer at the same time.